Recent developments in Complementary Metal Oxide Semiconductor (CMOS) devices have improved their radiation hardness, response linearity, readout noise and thermal noise, making them suitable for x-ray crystallography detectors. Large-format (e.g. 10 cm x 15 cm) CMOS sensors with a pixel size of 100x100 microns are now available that can be butted together on three sides so that very large area detector systems can be fabricated. They can be readout at up to 30 frames/sec. An important feature of CMOS systems is that they can be readout continuously without interrupting data collection. The large area format allows the scintillator converting film to be directly coupled via a thin, flat fiber optic plate to the CMOS sensor. This arrangement gives an excellent signal to noise ratio, a very linear response to x-ray dose, and a high dynamic range. We have developed a 6-module system with an active area of 28.2 x 29.5 cm (8.3 x 106 pixels) for use at synchrotron-based crystallography beamlines. Initial experiments have been carried out at ALS beamline 4.2.2. Data are typically recorded without an x-ray shutter, rotating the crystal sample continuously with an exposure time of 0.1 sec/frame and a rotation speed of 1 deg/sec. A data set of 1,800 data frames is typically recorded in 180 seconds, covering an angular range of 180 degrees. These data can be processed with any of the standard data analysis programs using 3-dimensional Bragg-spot profile analysis. The crystallographic results are typically better than equivalent data recorded on a CCD system, due to the 10X finer angular resolution of the recorded data. CMOS systems scale up in size rather easily. We are currently designing a 12-module system, with an array of 3,800 x 4,200 pixels: considerably larger than any existing CCD or pixel-array system.